1. Field of Invention
The invention relates to a fuel cell and a method of manufacturing the fuel cell. More specifically, the invention relates to an art for manufacturing a catalytic layer for an electrode which is in contact with an electrolyte in a fuel cell such as a polymer electrolyte fuel cell.
2. Description of Related Art
In general, a polymer electrolyte fuel cell is based on a structure of “an electrode-electrolyte conjugant” wherein an anode and a cathode are disposed on opposed surfaces of a polymer electrolyte. An electrode is usually composed of a catalytic layer and a gas diffusion layer and constructed such that the catalytic layer is in contact with the electrolyte.
In the thus-constructed fuel cell, when fuel gas (e.g. hydrogen) is supplied to the anode and oxidizer gas (oxygen gas) is supplied to the cathode, hydrogen ions generated in the anode move towards the cathode through the electrolyte and turn into water. By utilizing this electrochemical reaction, electric energy is taken out.
An electrode reaction for a fuel cell proceeds on an electrode catalyst. For example, in the case of a hydrogen-oxygen fuel cell, chemical reactions on the cathode side and the anode side can respectively be expressed as follows.
cathode side: ½O2+2H++2e−→H2O
anode side: H2→2H++2e−
As is apparent from the aforementioned formulas, the electrode reaction requires movements of electrons and ions. Thus, in order for a catalytic electrode to function as “a reaction field”, it is preferable that a catalytic activation substance be in contact with both an electron conductive substance and an ion conductive substance.
An electrode catalytic layer for a polymer electrolyte fuel cell is largely classified into the following three types.
<TYPE 1>
A carbon material (e.g. carbon black) is used as an electron conductive catalyst carrier. A catalytic activation substance such as platinum (Pt) is carried on the carbon material and mixed with an ion conductive substance (e.g. a polymer electrolyte).
<TYPE 2>
There is no catalyst carrier. Particles of a catalytic activation substance are mixed with an ion conductive substance.
<TYPE 3>
A layer of a catalytic activation substance such as Pt is directly provided on a surface of an electrolyte or a gas diffusion layer by means of plating or vaporization.
In TYPE 1 and TYPE 2, if occasion demands, a binder such as poly-tetra-fluoro-ethylene (PTFE) may further be included.
Among the aforementioned electrode catalytic layers, TYPE 1 is most commonly used because of the greatest specific surface area. As a rare case, it has also been reported that high outputs are achieved through combination of TYPE 1 and TYPE 3.
In a method of manufacturing a fuel-cell electrode having a catalytic layer of TYPE 1, a catalytic activation substance is first carried on an electron conductive substance to form a carrier-carrying catalyst. Then, the carrier-carrying catalyst is mixed with an ion conductive substance (if occasion demands, a binder is also added). Next, a layer of the mixture is formed on the surface of a gas diffusion layer or an electrolyte and finally bonded to a layer structure of the electrolyte/the catalytic layer/the gas diffusion layer.
In this case, the catalytic layer is not densely filled with the carrier-carrying catalyst and the ion conductive substance. The catalytic layer needs pores through which a gaseous reaction substance flows. Thus, the mixing ratio of the carrier-carrying catalyst and the ion conductive substance has a suitable range. However, within the range of the mixing ratio, it is difficult to cover all the surfaces of the carrier with the ion conductive substance.
Further, catalytic activation substances are homogeneously carried on the surface of the carrier. Therefore, as a matter of course, there are quite a few catalytic activation substances which are out of contact with the ion conductive substance. Even in the case where the catalytic activation substances are in contact with the ion conductive substance, if they are ion conductive substances separated from the electrolyte or if the carrier-carrying catalyst itself is separated from a network of electron conduction from the electrode to the terminal, they do not function as the electrode catalyst. Because of these reasons, the catalyst utilization ratio of the fuel-cell electrode having the catalytic layer of the structure of TYPE 1 is limited to approximately 20 to 70%.
Further, a fuel-cell electrode having the catalytic layer structure of TYPE 2 or TYPE 3 does not have a catalytic carrier. Therefore, the specific surface area (surface area per weight) of particles or layers of catalytic activation substances is small. Thus, a large quantity of catalyst is required to ensure a sufficient reaction area. For example, in the case of Pt catalyst, 2 mg or more of the catalyst is necessitated for an electrode area of 1 cm2.
Further, if the catalytic structures of TYPE 1 and TYPE 3 are combined, i.e., in the case of a fuel-cell electrode wherein a Pt catalytic layer is formed on the surface of an electrolyte and a catalytic layer of TYPE 1 is formed on the Pt catalytic layer, the electric power generation capability can be enhanced to some extent. Nevertheless, a large amount of catalyst is used, so that the catalyst utilization ratio is not necessarily favorable.